mmc: core: Enable / disable re-tuning

Enable re-tuning when tuning is executed and
disable re-tuning when card is no longer initialized.

In the case of SDIO suspend, the card can keep power.
In that case, re-tuning need not be disabled, but, if
a re-tuning timer is being used, ensure it is disabled
and assume that re-tuning will be needed upon resume
since it is not known how long the suspend will last.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>